$ strace -e open perl -e 'open $f, "+>", undef' 2>&1 | tail -n 1
open("/tmp/PerlIO_vfNALU", O_RDWR|O_CREAT|O_EXCL|O_LARGEFILE, 0600) = 3

##</code><code>##

$ perl -le 'open $f, "+>", undef;
print readlink "/proc/self/fd/" . fileno $f'
/tmp/PerlIO_vJg2Uf (deleted)

##</code><code>##

$ perl -le 'open $f, "+>", undef;
print readlink "/proc/self/fd/" . fileno $f'
/tmp/.nfs0004821b00000353